The present invention generally relates to propagation path estimation methods for a multi-stage type interference canceler applied to a DS-CDMA mobile communication and to interference elimination apparatuses, and more particularly to a propagation path estimation method for a multi-stage type interference canceler that eliminates interference by estimating a characteristic of a propagation path using a pilot symbol transmitted in a channel different from a data channel and subtracting a generated interference replica from a received signal, and to an interference elimination apparatus that eliminates the interference in such a manner.
In a DS-CDMA (Direct Sequence Code Division Multiple Access) mobile communication system, interference caused by multipath from other mobile stations (other user channels) is generated due to the cross correlation among spreading codes introduced by asynchronization between the mobile stations. Such interference causes the transmission quality and the channel capacity of the mobile communication system to deteriorate. For this reason, there is a need to realize an interference canceler, which can eliminate the interference from the received signal with satisfactory accuracy and to improve the signal-power-to-interference-power ratio (SIR).
FIG. 6 shows a diagram showing a conventional multi-stage type interference canceler. Each stage of the multi-stage type interference canceler includes interference canceler units (ICU) 51 and a combining unit (xcexa3) 52. As can be seen, such stages are successively connected in series. FIG. 6 shows a case which the multi-stage type interference canceler includes a first stage 84, second stage 86, through mth stage 88. Further, data symbol receivers 53 are included in the final mth stage 88.
The interference canceler units 51 and the receivers 53 of the final stage are provided in parallel in correspondence with user channels. Subscripts to designations ICU1,l-ICU1,k, ICU2,l-ICU2,k, . . . of the interference canceler units 51 indicate the stage number and the user number corresponding to the user channel. Similarly, subscripts to designations ReCm,l through ReCm,k of the receivers 53 indicate the stage number and the user number corresponding to the user channel.
In the first stage 84, a received signal Ro is input to each of the interference canceler units ICU1,l-ICU1,k corresponding to the user channels. The interference canceler units ICU1,l-ICU1,k respectively output symbol replica signals S1,l-S1,k and interference replica signals d1,l-d1,k. The combining unit 52 combines the interference replica signals d1,l-d1,k corresponding to the user channels to obtain a combined signal, and then outputs an error signal el of the first stage 84 by subtracting the combined signal from the received signal Ro.
In the second stage 86, the error signal e1 from the combining unit 52 of the first stage 84 and the symbol replica signals S1,l-S1,k from the interference canceler units ICU1,l-ICU1,k of the first stage are respectively input to the interference canceler units ICU2,l-ICU2,k. The interference canceler units ICU2,l-ICU2,k respectively output symbol replica signals S2,l-S2,k and interference replica signals d2,l-d2,k. The combining unit 52 combines the interference replica signals d2,l-d2,k corresponding to the user channels to obtain a combined signal, and then outputs an error signal e2 of the second stage 86 by subtracting the combined signal from the error signal el of the first stage 84.
In the final mth stage 88, an error signal emxe2x88x921 of a preceding (mxe2x88x921)th stage and symbol replica signals Sm-1,l-Sm-1,k from the preceding (mxe2x88x921)th stage are respectively input to the receivers ReCm,l-ReCm,k. The receivers ReCm,l-ReCm,k then eliminate the interference from these input signals so as to decode the data symbol. By successively repeating the interference elimination process at each of the stages, the error signals gradually become smaller. Therefore, it is possible to obtain symbol replica signals without interference among the users or the like.
FIG. 7 is a diagram showing a conventional interference canceler unit. The interference canceler unit (ICU) 51 includes despreading processors 61, a combining unit 62, a decision unit 63, spreading processors 64, and a combining unit 65. The despreading processors 61 each include a despreader 61-1, an adder 61-2, a multiplier 61-3 and a propagation path estimation circuit 61-4. The spreading processors 64 each include a multiplier 64-1, an adder 64-2 and a respreader 64-3.
The number of despreading processors 61 and spreading processors 64 respectively correspond to the number of received delayed waves, that is, the number of the paths (propagation paths) to be multiplexed. FIG. 7 shows a case where three despreading processors 61 and three spreading processors 64 are provided in parallel. In FIG. 7, a subscript i (in this example, i=1 to 3) indicates the signals corresponding to the different paths. The signals corresponding to the different paths are often referred to as RAKE fingers.
An error signal ejxe2x88x921 from a preceding stage (the received signal Ro in the case of a first stage) and symbol replica signals Sjxe2x88x921,l-Sjxe2x88x921,k of the preceding stage (zero in the case of the first state) are input to the despreading processor 61, where j denotes the stage number. The despreader 61-1 carries out a despreading and demodulation with respect to the error signal ejxe2x88x921 of the preceding stage (the received signal Ro in the case of the first stage) using a spreading code.
The despread and demodulated signal, and one symbol replica signals Sjxe2x88x921,l-Sjxe2x88x921,k of the preceding state (zero in the case of the first stage) are combined by the adder 61-2 to produce a received symbol Ri. The received symbol Ri is then input to the propagation path estimation circuit 61-4. The propagation path estimation circuit 61-4 estimates the characteristic of the corresponding propagation path using a pilot symbol shown in FIG. 7 and then outputs a propagation path estimation value "xgr"i{circumflex over ( )} for each path.
By multiplying a complex conjugate "xgr"i{circumflex over ( )}* of the propagation path estimation value "xgr"i{circumflex over ( )} to the signal Ri in the multiplier 61-3, a received symbol is produced, which is eliminated of a phase error caused by the effects of the propagation path.
The output signals of the multipliers 61-3 for each of the paths are subjected to a diversity composing in the combining unit (xcexa3) 62. A diversity combined received symbol xcexa3Ri"xgr"i{circumflex over ( )}* is compared with a threshold value in the decision unit 63, where a data symbol is provisionally decided.
The signals generated and output from the despreading processors 61 will be referred to as replica generation signals. The replica generation signals are converted into symbol replica signals and interference replica signals in the spreading processors 64, which are then transmitted to the next stage.
A provisionally decided data symbol Zŝ output from the decision unit 63 is branched in correspondence with the different paths. Further, the propagation path estimation value "xgr"i{circumflex over ( )} is multiplied by the multiplier 64-1 of each of the spreading processors 64. Therefore, the provisionally decided data symbol Zŝ is again decomposed into the signals corresponding to the channels, and transmitted to the next stage as symbol replica signals Sj,l-Sj,k.
In addition, the symbol replica signals Sj,l-Sj,k corresponding to each path are output from the multiplier 64-1 and one of the symbol replica signals Sjxe2x88x921,l-Sjxe2x88x921-,k from the preceding stage are input to the adder 64-2. The adder 64-2 outputs the difference between one of the symbol replica signals Sj,l-Sj,k of this stage and one of the symbol replica signals Sjxe2x88x921,l-Sjxe2x88x921,k of the preceding stage. The output signal of the adder 64-2 is then spread by the respreader 64-3 using the spreading code. A spread output signal from the respreader 64-3 is combined with spread output signals from the respreaders 64-3 of the other paths in the combining unit 65. A combined output from the combining unit 65 is then transmitted to another such as the second combining unit 52 shown in FIG. 6, as interference replica signals dj,l-dj,k.
In FIG. 8, the receiver unit 53 of the final stage is shown. The receiver 53 includes the despreading processors 61, the combining unit 62 and the decision unit 63. Symbol replica signals Smxe2x88x921,l-Smxe2x88x921,k and an error signal emxe2x88x921 from an interference replica generation unit of the preceding stage are input to the despreading processors 61 of the receiver 53 in the final stage. The despreading processors 61 in the receiver 53 of the final stage carry out a process similar to that of the despreading processor of the interference canceler unit described above, and outputs a demodulated symbol.
The combining unit 62 of the receiver 53 in the final stage carries out diversity combining with respect to the demodulated symbols output from the despreading processors 61. Further, the decision unit 63 makes a final decision with respect to a diversity demodulated symbol xcexa3Ri"xgr"i{circumflex over ( )}*, and reproduces it as information data. This data is then output to a decoder Ro, where decoding process such as deinterleaving and error correction is performed.
Accordingly, the received signal is subjected to a despreading process for each delayed wave (path) in the despreading processor 61 of the receiver in the final stages of the interference replica generation unit for each stage corresponding to each user channel. Further, a signal corresponding to each path is converted into a symbol rate.
The propagation path estimation circuit 61-4 estimates the characteristic (fading complex envelope) of each path using the pilot symbol. The propagation path estimation circuit also multiplies the complex conjugate to detect the received data by generating the data symbol in which the effects of the propagation path is eliminated.
Further, a description will be given of the estimation of the propagation path using the pilot symbol. Generally, in mobile communications, the propagation path characteristic changes due to the fading as a communication terminal moves in an environment where multipaths are produced.
In a case where the data symbol is received and demodulated in such an environment, a generally employed technique receives and demodulates the pilot symbol transmitted along with the data symbol. Further, this technique estimates the propagation path characteristic (fading complex envelope) from the pilot symbol and coherently detects the data symbol by eliminating the effects of the propagation path.
The pilot symbol is a known symbol having a predetermined amplitude and phase. The pilot symbol is either inserted between data symbols being transmitted or is transmitted in a channel different from the data symbol channel.
In the case where the pilot symbol is inserted between the data symbols, the pilot symbol is inserted at a predetermined position in the data frame and is then transmitted. At a receiving unit, a synchronization is performed using a preamble or the like added in front of the data frame, so as to recognize the pilot symbol position. The symbol at this position is demodulated, and then a propagation path characteristic is estimated from the values of the amplitude and phase of the demodulated pilot symbol.
On the other hand, in the case where the pilot symbol is transmitted in a channel different from the data symbol channel, the pilot symbol and the data symbol are multiplexed and then transmitted in mutually orthogonal channels. Because the pilot symbol is transmitted in parallel with the data symbol, this method is referred to as a parallel pilot channel system. Since the parallel pilot channel system includes multiplexing and demultiplexing using an orthogonal code, studies are being made for application to mobile communications using the DS-CDMA.
For the sake of convenience, the symbol transmitted as the pilot symbol will be denoted by Z. In this case, if the propagation path characteristic id denoted by "xgr", the received symbol becomes Z "xgr". The predetermined amplitude and phase of the pilot symbol Z is known. Therefore, when a complex conjugate Z* of the known pilot symbol Z is multiplied with the received symbol Z "xgr", which is received via the propagation path, the product takes a value "xgr" |Z|hu 2.
Because the magnitude of the pilot symbol vector is known (|Z| may be assumed to be 1), the propagation path characteristic "xgr" can be estimated by a calculation. The propagation path estimation circuit 61-4 carries out this calculation and outputs the propagation path estimation value "xgr"{circumflex over ( )}. The propagation path estimation value "xgr"{circumflex over ( )} is described by the following formula (1).
"xgr"{circumflex over ( )}=Z"xgr"xc2x7Z*="xgr"xc2x7|Z|2xe2x80x83xe2x80x83(1)
Since the received symbol is actually affected by noise and interference, it is difficult to accurately estimate the propagation path characteristic. Accordingly, propagation path characteristics obtained from a plurality of pilot symbols are averaged, so as to improve the accuracy of the estimation. Generally, in order to follow the change in the propagation path with time due to fading, a moving average is obtained from among a plurality of pilot symbols in a moving duration.
A propagation path characteristic estimated from an average of a plurality of pilot symbols before and after a nth pilot symbol will be denoted by "xgr"n{circumflex over ( )}. If the nth transmitted data symbol is denoted by Zn and the actual propagation path is denoted by "xgr"n, the received data symbol becomes Znxc2x7"xgr"n. Thus, by multiplying the complex conjugate "xgr"n{circumflex over ( )}* of the propagation path estimation value by the received data symbol Znxc2x7"xgr"n and dividing the product by a square of the absolute value of the vector of the propagation path estimation value "xgr"n{circumflex over ( )}, it is possible to demodulate from the received data symbol Znxc2x7"xgr"n, the transmitted data symbol Zn from which the effects of the propagation path are eliminated. A demodulated data symbol Zn, which is obtained in the above described manner is described by the following formula (2).
Zn{circumflex over ( )}=Znxc2x7"xgr"nxc2x7"xgr"n{circumflex over ( )}*/|"xgr"n{circumflex over ( )}|2xe2x80x83xe2x80x83(2)
The date symbols obtained by the coherent detection described above are subjected to the diversity combining in the combining unit 62, and then the decision unit 63 decides the phase thereof. In addition, in the case of a multi-level QAM or the like, the decision unit 63 also decides the amplitude thereof.
In the spreading processor 64, the propagation path estimation value "xgr"{circumflex over ( )} described above is multiplied to the symbols, which are decided in the decision unit 63. The symbols are then branched to the corresponding paths again before respreading is performed. As a result, symbol replica signals and interference replica signals are generated which are transmitted to the next stage. In the next stage, a process similar to that described above is carried out, and is then repeated in the subsequent stages, so that the interference is gradually eliminated.
Although, only one signal line is illustrated in FIG. 7, a bus structure is actually used. In other words, the data symbol and the pilot symbol are multiplexed and then transmitted.
FIG. 9 shows a timing relationship of the propagation path estimation using the pilot symbol and the demodulated data symbol. As shown in FIG. 9, the pilot symbol and the data symbol are respectively transmitted in a pilot channel 71 and a data channel 72, which are independent.
In this case, the data symbol and the pilot symbol are spread and multiplexed using independent codes, and modulated by the same carrier. For this reason, the two channels are received after being subjected to the effects of the same propagation path. At the receiving end, the despreading is carried out using the respective codes, so as to separate the data symbol and the pilot symbol.
In order to reduce the effects of the interference and noise, the propagation path estimation value used for the demodulation process is obtained by averaging the propagation path estimation values using the pilot symbols in a predetermined duration 73, as shown in FIG. 9. In addition, the moving average is obtained in a state where the predetermined duration 73 is maintained, so as to follow the change in the propagation path due to fading.
The moving average duration 73 is determined within a range such that the propagation path characteristic does not change greatly. As shown in FIG. 9, in order to avoid a processing delay, the propagation path estimation is made from the moving average of the pilot symbols before the data symbol is demodulated, and a demodulated data symbol 74 is subjected to the coherent detection using the propagation path estimation value.
FIG. 10 shows a timing relationship of the propagation path estimation in each stage of a multi-stage type interference canceler and the demodulated data symbol In FIG. 10, the timing relationship of a frame header symbol 81, a moving average duration 82 for estimating the propagation path, and a demodulated data symbol 83 is shown in (A) with respect to the first stage, in (B) with respect to the second state and in (C) with respect to the third stage.
Between the stages, an inter-stage processing delay time xcfx84 on the order of approximately several symbols occurs. This inter-stage processing delay time xcfx84 is introduced by the delay adjustment for carrying out the RAKE combining (diversity combining), the input/output delay when transferring the data between the stages, and the like.
The reception characteristic of the pilot coherent detection is greatly affected by the estimation accuracy of the propagation path. Particularly in the case of the multi-stage type interference canceler, the propagation path estimation value is not only used for the RAKE combining in order to make the provisional decision, but also for generating the interference replicas. Thus, the propagation path estimation value greatly affects the interference elimination characteristic. Accordingly, in a DS-CDMA mobile communication, it is important to improve the estimation accuracy of the propagation path, in order to improve the reception characteristic and cell capacity.
As a method of improving the estimation accuracy of the propagation path, it is conceivable to also use the pilot symbol after the data symbol is demodulated. However, this method would increase the processing delay at each stage. Particularly in the case of an interference canceler having a large number of stages, the processing delay is a multiple of the number of stages, which introduces problems thereby. On the other hand, in the DS-CDMA mobile communication if a transmission power control is carried out, the reception characteristic further deteriorates because the increase in the processing delay causes a delay of the transmission power control.
Therefore, an object of the present invention, is to improve the estimation accuracy of the propagation path in a multi-stage type interference canceler applied to a DS-CDMA mobile communication, without increasing the processing delay when making a coherent detection in the parallel pilot channel system.
These and other objects are met in accordance with the present invention directed to a propagation path estimation method for estimating a characteristic of a propagation path using a pilot symbol. The pilot symbol is transmitted in a channel different from a data symbol channel in a multi-stage type interference canceler that receives a signal modulated by a spreading code. The method according to the present invention includes notifying a propagation path estimation value estimated at each stage of the interference canceler to another stage. The method further includes calculating the propagation path estimation value of each stage by using the propagation path estimation value estimated at each stage and the propagation path estimation value notified from another stage.
The propagation path estimation method according to the present invention also includes notifying the propagation path estimation value estimated at a latter stage to a preceding stage, and calculating the propagation path estimation value at the preceding stage using the propagation path estimation value estimated at each stage and the propagation path estimation value notified from the latter stage.
In the multi-stage type interference canceler, the estimation accuracy of the propagation path improves at the latter stages because the interference is further eliminated in the latter stages. Hence, by using the propagation path estimation value estimated at the latter stage in the preceding stage, it becomes possible to improve the estimation accuracy of the propagation path at the preceding stage.
A propagation path estimation method according to the present invention includes notifying a propagation path estimation value previously estimated in a preceding stage to a latter stage, and calculating the propagation path estimation value at the latter stage using the propagation path estimation value estimated at each stage and the propagation path estimation value notified from the preceding stage.
Since a processing delay on the order of approximately several symbols occurs between the stages, the latter stage carries out a demodulation process with respect to the symbol, which is input several symbols before as compared to the symbol presently input to the preceding stage. Accordingly, the propagation path estimation value at a time position after a symbol presently input to the latter stage is already estimated at the preceding stage. Hence, by using the propagation path estimation value already estimated at the preceding stage, it is possible to improve the estimation accuracy of the propagation path at the latter stage.
A propagation path estimation method according to the present invention includes calculating a propagation path estimation value from a weighted average of the propagation path estimation value estimated at each stage and the propagation path estimation value notified from another stage, depending on a reliability of each stage.
A multi-stage type interference elimination apparatus according to the present invention eliminates interference of a signal modulated by a spreading code. The apparatus includes an interference canceler unit at each stage and a receiver at a final stage, which are coupled in series. A propagation path estimation circuitry is provided in both the interference canceler unit of each stage and the receiver at the final stage. The propagation path estimation circuit estimates a characteristic of a propagation path using a pilot symbol transmitted in a channel different from a data symbol channel. The propagation path estimation circuit further notifies a propagation path estimation value estimated at each stage to another stage via a signal line, and at each stage calculates the propagation path estimation value using the propagation path estimation value estimated in its own stage and the propagation path estimation value provides from another stage. An interference elimination apparatus according to the present invention is configured so that the propagation path estimation circuit notifies the propagation path estimation value estimated at a latter stage to the preceding stage. Further, the propagation path estimation circuit at the preceding stage calculates the propagation path estimation value using the propagation path estimation value estimated in its own stage and the propagation path estimation value notified from the latter stage.
An interference elimination apparatus according to the present invention is constructed so that the propagation path estimation circuit provides a propagation path estimation value previously estimated in a preceding stage to the latter stage. Further, the propagation path estimation circuit at the latter stage calculates the propagation path estimation value using the propagation path estimation value estimated in its own stage and the propagation path estimation value from the preceding stage.
An interference elimination apparatus according to the present invention is configured so that the propagation path estimation circuit calculates a propagation path estimation value from a weighted average of the propagation path estimation value of each stage and the propagation path estimation value from another stage, depending on a reliability of each stage.